JUNE 27, 1912] 
NATURE 
437 
power of prisms of different kinds of glass to be pro- 
portional to their refractive power, involving the im- 
possibility of ever obtaining an achromatic lens. Even 
after a hundred years the Newtonians out-Newtoned 
Newton in their antipathy to anything that seemed 
counter to his views; and their hostility to Thomas 
Young’s doctrine of interference is a matter of history. 
Christiaan Huygens, Newton’s great contemporary, 
propounded his wave-theory of light in 1678, though 
his famous * Traité de la Lumiére ” appeared only in 
i690. Few British students have ever read that rare 
work; but none can read it without being impressed 
with the genius of its author. Everyone knows of 
Huygens as the inventor of the wave-theory of light; 
but how few are familiar with the contents of the 
treatise! He expounds the analogy of the propagation 
of light with that of sound, then points out the essen- 
tial differences, and develops the geometrical notion of 
movements spreading in spherical waves. He had, in 
fact, to take into account six fundamental facts :—(1) 
The rectilinear propagation of light; (2) the mutual 
penetrability of two beams where they cross one 
another; (3) the law of reflection; (4) the law of re- 
fraction (which he had learned from Descartes); (4) 
atmospheric refraction; (5) the finite speed of light, 
discovered by. Roemer in 1676; and (6) the double- 
refraction of Iceland spar, discovered by Bartholinus 
in 1669. 
The insight with which, by aid of. his conception 
of elementary waves building up an enveloping wave- 
front, Huygens succeeded in giving a consistent 
theory, is a matter for wonder and admiration. He 
availed himself of Fermat’s principle of least time, 
deduced from it the law of sines for refraction, and 
based on it the geometrical construction for his wave- 
fronts which now appears in all books on physical 
optics. It is true that he had no conception of trans- 
versality in the movements of his waves, or of the 
principle of interference, or even of the existence of 
trains of waves or of wave-length. His wave-theory 
was far from being the complete doctrine of Young 
and Fresnel, and belongs to geometrical rather than 
to physical optics. But the exquisite skill with which 
he unravelled the intricacies of double-refraction in 
crystals and the anomalies of atmospheric refraction 
must excite the admiration of every reader. His 
speculations as to the ether of space, his suggestive 
views of the structure of crystalline bodies, and his 
explanation of opacity, slight as they are, surprise 
one by their seeming modernness. He detected the 
double-refraction of quartz, and discovered the pheno- 
menon of polarisation, while frankly unable to explain 
it. Another section of his book deals with aspherical 
forms of lenses for focussing light when one surface 
is prescribed. 
ABERRATIONS. 
The enormous focal lengths adopted by Huygens 
for his telescopic object glasses arose from their com- 
parative freedom from aberrations. No actual lens 
ever gives perfect stigmatic results; and every be- 
ginner knows that aberrations are of two classes: 
those that arise from the polychromatic nature of 
light, and those which, even when monochromatic 
light is used, are due to the form of the surface of 
the lens, and are often—though not very happily— 
termed spherical aberrations. Newton calculated 
(‘“ Opticks,” pp. 84, 89) that in a 100-foot telescope 
with suitable aperture the aberration of colour would 
be at least 1200 times as great as the aberration 
caused by the sphericity of figure of the object glass. 
We know, in fact, that in despair at making a lens 
devoid of chromatic aberration, he gave up refractors 
and invented his reflecting telescope. But when in 
1757 Dollond, by the invention of the achromatic lens, 
NO. 2226, vor. 89] 
removed the worst of the aberrations, the correction 
of the aberrations due to form became the next desir- 
able step. Descartes, Deschales, and other writers 
suggested various devices for grinding lenses with 
hyperbolic, elliptical, and other aspherical curves; but 
practical difficulties prevented their use. In the early 
part of the nineteenth century, Coddington and Airy, 
the younger Herschel, and others investigated in 
great detail the aberrations of lens combinations, and 
brought that part of optics to a high pitch, though 
much of their work remained unknown outside Eng- 
land. 
ILLUMINATION, 
During the past seven years there has been great 
activity in the development of the branch of geo- 
metrical optics concerned with illumination, involving 
questions of the distribution of light, and the measure- 
ment of it in quantity and intensity by photometers. 
Though a better standard source of light than either 
the Harcourt Pentave lamp or the Hefrer amyl-acetate 
lamp is still a desideratum, it is satisfactory to know 
that international agreement upon the unit of light is 
practically secured, through the collaboration of the 
four great laboratories at Sevres, Charlottenburg, 
Bushy, and Washington. Committees have been 
actively at work on the questions of minimum illu- 
mination required in schools, libraries, factories of 
various kinds, and in roads and streets. Even the 
House of Commons has awakened to the fact that 
the illumination enjoyed by its members is only about 
half a candle-foot, whereas for comfortable reading it 
should be two or three times that amount. Photo- 
metry has indeed grown since the photometric law of 
inverse squares was first announced by Deschales in 
1674, or since the early treatises of Bougner and Lam- 
bert. New forms of photometer have multiplied, and 
every month sees fresh developments. 
PuysicaL Optics. 
Wien we turn to the vast subject of physical optics 
we cannot but be struck with the variety of pheno- 
be taken into account by 
mena which must 1 
anyone who would deal with the nature of 
light itself, or with the mechanism of the 
ethereal medium by which it is conveyed. Dispersion * 
and its anomalies, interference, diffraction, the mulfi- 
tudinous effects of polarisation, the problems of radia- 
tion and luminescence, of opulescence, and the blue of 
the sky, of iridescence, and the gorgeous colours of 
butterflies and humming-birds, to say nothing of 
radio-activity, or of the chemical, physiological, elec- 
trical, magnetic, and mechanical relations of light, 
furnish whole fields in which knowledge is still in the 
making. ; 
In physical optics, though there are mathematical 
laws, such as those discovered by Fresnel and Stokes, 
to be mastered, the chief concern is with physical 
phenomena; and the study of these would seem to 
be inseparable from speculations as to the nature of 
the luminiferous zther, and from consideration of the 
conflicting theories as to its constitution. Formerly 
the vexed question was the mechanical explanation of 
an zther which should behave like an elastic solid a 
million times more rigid than steel, and at the same 
time as a mere vapour a billion times less dense than 
air. Then there was an outstanding quarrel between 
the followers of Fresnel and those of Neumann and 
McCullagh as to whether the vibrations of light were 
2 Herschel, in 1828, in his article ‘‘On Light” (Encyclop. Metrop., 
Tr. 450), declared : ‘* The fact is that neither the corpuscular nor the undula- 
tory, or any othier system which has yet been devised, will furnish that com- 
plete and satisfactory explanation of @// the phenomena of light which is 
desirable. Certain admissions must be mzd, at every step, as to modes of 
mechanical action, where we are in total ignorance of the acting forces ; and 
we are called on, where reasoning fails us occasionally for an exercise of 
faith.” 
